Method of graphitizing fibers

ABSTRACT

A method for carbonizing and/or graphitizing precursor fibers selected from the group consisting of polyacrylonitrile and aromatic polyamide fibers, wherein the fibers are pretreated with an oxygen-containing atmosphere at 180*-550*C. and thereafter heated in a laser beam in a non-oxidizing atmosphere at 700*1,200*C. for carbonizing and at 1,200*-3,600*C. for graphitizing.

United States Patent Binning et al.

[ 1 Oct. 17, 1972 METHOD OF GRAPHITIZING FIBERS Inventors: Robert C. Binning, Kettering; Leo P. Parts, Dayton; Robert J. Peresie, Brecksville; Margaret L. Rodenburg, Kettering, all of Ohio Monsanto Research Corporation, St. Louis, Mo.

Filed: Sept. 6, 1968 Appl. No.: 757,964

Assignee:

US. Cl. ..423/447, 423/448, 204/157 Int. Cl. ..C0lb 31/07 Field of Search ..23/209. l 209.3, 209.4;

References Cited UNITED STATES PATENTS 12/1961 Soltes ..23/209.1 X 11/1966 Tsunoda ..23/209.1 8/1968 I-Iough et a1. ..23/209.3 X 6/1969 Stuetz ..23/209.1 6/1963 Hill et a1. ..260/78 Sharkey et al. Nature Vol. 202, June 6, 1964, pages 988- 989 Smith et al. The Laser" Copyright 1966 by Mc- Graw- Hill, Inc., page 460 Primary Examiner-Edward J. Meros Attorney-L. Bruce Stevens, Jr.

[57] ABSTRACT A method for carbonizing and/or graphitizing precursor fibers selected from the group consisting of po1yacry1onitri1e and aromatic polyamide fibers,

wherein the fibers are pretreated with an oxygen-containing atmosphere at 180-550C. and thereafter heated in a laser beam in a non-oxidizing atmosphere at 700l,200C. for carbonizing and at 1,2003,600 C. for graphitizing.

6 Claims, 2 Drawing Figures IIQMQLM ugh METHOD OF GRAPHITIZING FIBERS BACKGROUND OF THE INVENTION The invention pertains to a process for preparing carbonized and/or graphitized fibers by heating suitable precursors in a laser beam, and particularly provides a continuous process for carbonizing or graphitizing a precursor yarn.

Previously, carbonized or graphitic fibers have been prepared by thermal degradation of variousfibcrs, e.g., cellulose, polyacrylonitrile, aromatic polyamide, etc. (see Ezekiel and Spain, Preparation of Graphite Fibers from Polymeric Fibers, Journal of Polymer Science, Part C, No. 19, pp. 249-265 (1967)). The process of pretreating an acrylonitrile precursor by heating in an oxygen-containing atmosphere and thereafter carbonizing at 7001200C. and graphitizing at l200-3600C. was disclosed by Tsunoda in US. Pat. No. 3,285,696, issued Nov. 15, 1966. A process for continuously graphitizing a carbonaceous thread by passing an electric current through it to heat it is disclosed by Cranch and Shinko in US. Pat. No. 3,313,597, issued Apr. 11, 1967.

- SUMMARY OF THE INVENTION An object of this invention is to provide a process for preparing flexible carbonized or graphitized fibers by a process utilizing a laser beam, said fibers being useful in reinforcing plastic composites.

These and other objects hereinafter defined are met by the invention wherein there is provided a method for preparing a carbonized fiber from a precursor fiber selected from the group consisting of l) acrylonitrile polymer and (2) an aromatic polyamide consisting of repeating units represented by the general formula:

wherein R and R are selected from the group consisting of hydrogen, lower alkyl of up to three carbon atoms, phenyl, lower alkoxy containing up to three carbon atoms and nitro, and wherein the R groups can be the same or different and the R groups must be the same, and wherein X and Y are selected from the group consisting of hydrogen, lower alkyl containing up to three carbon atoms and phenyl, the phenylene radicals of said general formula being oriented other than ortho, which comprises (a) pretreating the fiber by heating at a temperature of from 180 to 550C. in an oxygen-containing atmosphere for a time sufficient to blacken the fiber, and thereafter (b) heating in a laser beam in a non-oxidizing atmosphere at a temperature between 700 and about 1,200C. for a period of time greater than one-tenth of a second, said time being dependent upon the temperature. The present method for preparing graphitized fibers comprises the additional step of (c) heating in a laser beam in a non-oxidizing atmosphere at a temperature between about 1,200 and 3,600C. for a period of time greater than one-tenth of a second, said time being dependent upon the temperature.

It is well known that carbon-base fibers are useful in reinforced plastic composites (see Schmidt and Jones, Carbon-Base Fiber Reinforced Plastics, Chemical Engineering Progress, Vol. 58, No. 10, pp. 42-50 (1962)). For such purposes it is desirable that the fibers be flexible and have a high tensile strength and elastic modulus. Generally, graphitized carbon fibers are preferred over carbonized fibers for their greater mechanical strength, higher modulus, and higher thermal stability.

It is further known that the term carbon-base fibers includes a wide range of materials varying in chemical composition within the range -100 percent carbon, and varying considerably in crystal structure, e.g., from a highly disordered or essentially amorphous structure in the carbonized fibers to a more ordered but not highly crystalline structure characterizing the graphitized fibers (see Franklin, The Structure of Graphitic Carbons, Acta Crystallographica, Vol. 4, pp. 253-261 (1951)) and representing a point in the continuum from amorphous carbon to highly crystalline three-dimensionally ordered graphite.

The term carbonized fibers isv used herein for fibers containing at least 90 percent carbon, but showing essentially no (002) reflection of graphite by X-ray diffraction analysis. Graphitized fibers refers to fibers containing at least percent carbon and showing at least some degree of ordering .by X-ray diffraction analysis, e.g., the (002), (004) and reflections of the graphitic carbons. Generally, such fibers do not show the highly ordered structure of crystalline graphite.

The present process for carbonizing or graphitizing fibers generally yields fibers having tensile strengths of over 100 X 10 p.s.i. and elastic moduli of over 20 X 10 p.s.i. Such fibers find ready application in plastic composites for structural members, filament-wound tanks, ablative nose cones, rocket exhaust nozzles, electric brushes, etc., where they may be employed with epoxy, phenolic, silicone, polyimide, and other resin systems.

Novel energy sources, lasers, have become available during the recent years. Some types of lasers, e.g., the CO laser, convert electrical energy with high efficiency to intense, collimated beams of electromagnetic radiation. The laser output beams can be readily focused with great efficiency onto objects for generating high temperatures.

The improvement of the present invention over older methods of carbonizing and graphitizing fibers lies in the use of laser radiation for effecting the chemical and physical changes in the fibers. The lasers offer advantages for this operation as compared with conventional methods of heating, viz. l convenience in startup and shut-down without significant time lag, (2) rapid response and sensitive control of power output by simple optical, electronic, and electrical means, and (3) efficient energy utilization and optical manipulation inherent in utilizing a collimated beam of coherent radiation for heating. The beam is readily manipulated by optical means so as to generate desired energy flux densities and density gradients, using lenses or reflectors. The temperature of fibers exposed to the radiation is readily controlled either by changing the energy output of the laser or by changing the energy flux density in the irradiated zone by optical means. Laser heating permits carbonizing and graphitizing fibers to be conducted as-a continuous process. Fast production rates can be attained by the use of lasers which provide high power output.

According to the invention, precursor fibers which are preferably either acrylonitrile homopolymer or copolymers, or. an aromatic polyamide (previously disclosed in U.S. Pat. No. 3,232,9l0, issued Feb. 1, 1966), are first pretreated by heating in an oxygen-containing atmosphere at between 180 and 550C. for a time sufficient to. partially oxidize and blacken the fibers, and are then carbonized by heating in a laser beam in a nonoxidizing atmosphere at between 700C. and about l,200C. The time of exposure of the fibers to the laser beam during carbonizing varies with the temperature,

being about three minutes at l,060C. and somewhat longer at 700C. The optimum time is readily determined by experimentation so that carbonized fibers are thereby produced having suitably high carbon content and satisfactory physical properties.

As precursor fibers, various carbonaceous materials may be employed in addition to the preferred acrylonitrile polymer or aromatic polyamide aforementioned. Thus, cellulose in either its natural or regenerated form, e.g., rayon, may be used; likewise copolymers of acrylonitrile with up to mol percent of a-monovinyl compound such as methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidine chloride, 2-methyl-2-vinyl pyridine, etc. Catalysts may be incorporated in the fiber, to lower the decomposition temperature; e.g., in cellulose fibers there may be used ammonium phosphates, boric acid, zinc chloride, etc. The fibers may be used as single fibers or as monofilaments, or loose bundles or in the form of roving. For a continuous process thepreferred form is yarn. Other forms which are adaptable to the process are tape, woven fabric, matted fibers, paper, etc.

For convenience, the precursor fibers may be heated in the pretreating step in. the oxygen-containing atmosphere by conventional, relatively low-temperature, heating methods; viz. resistance-heating, flame-heating, radiative-heating, etc. well-known in the art. The pretreating may be done in bulk, in a batch operation, or in a continuous operation.

As a further description of the invention, precursor fibers which have been pretreated and carbonized, preferably but not necessarily as described above using laser irradiation, are graphitized by heating in a laser beam in a non-oxidizing atmosphere at between about l,200 and 3,600C. The time of exposure of the fibers to laser radiation during graphitizing varies with the temperature, being about 3 minutes at 2,27 0C. The optimum time is readily determined by experimentation. Excessive exposure at high temperatures (above 3,000C.) is to be avoided, as causing weakening of the fibers through sublimation of the graphitized carbon.

In order that the carbonized fibers may be heated at I temperatures above 1,200C.they are preferably surrounded by a non-oxidizing atmosphere such asnitrogen, hydrogen, helium, methane, etc. or mixtures thereof. However, for purposes of facilitating the carbonization or graphitization of the organic fibers, small amounts of air, chlorine, hydrogen chloride, etc. may be added as desired. Furthermore,-it may be desirable to perform one or more of the operations in a partial or full vacuum or even under increased pressure, any of which are permitted through the convenience of laser irradiation.

It is advantageous to heat fibers'under tension, to stretch them, and thereby orient the microcrystalline regions for attaining superior strength. Means may be provided for rotating the fibers around their longitudinal axis.

In addition to laser-irradiative heating during the carbonizing and graphitizing steps, it may also be employed for the pretreating or oxidation step. In order to attain desired temperature gradients during the heating, a plurality of laser beams may be employed, or a single laser beam may be split by the use of partly transmitting beam-splitters.

Although the process is admirably adapted to the continuous preparation of carbonized or graphitized yarn, other products may be produced which are novel, utilizing the unique source of energy provided by the laser beam. Thus, only one or more segments of a precursor yarn may be carbonized or graphitized, leav- I ing the ends attached to the original unchanged yarn. In another application, a surface of a bonded fibrous body may be graphitized without affecting its interior structure; such a graphitized surface may provide a useful bearing surface having lubricating properties. In another application, a fibrous mat, e.g., a sheet of paper, or a woven cloth may be exposed to a laser beam so as to carbonize precise areas for decorative or aesthetic purposes in various degrees of blackening, to simulate black or grey tones of a charcoal drawing, by suitably controlling the energy of the beam and the time of exposure at each area. In a similar manner, paper or cloth may be carbonized in very specific and narrowly defined areas, to produce symbols, letters, figures, bits of information, etc. for the purpose of communication.

BRIEF DESCRIPTION OF THE DRAWING- Some of the novel features of the present invention will become apparent from the following description which is to be considered in connection with the accompanying drawing wherein:

FIGS. 1 and 2 are representations of two embodiments of a continuous process for carbonizing or graphitizing organic fibers by laser-heating.

In FIG. 1 there is shown a laser beam, focused by a condensing lens, impinging upon a .yarn of organic fibers. The laser beam 1 emanates from Co -laser tube 2 through infrared radiation-transmitting lens 3. It passes through water-cooled germanium condensing lens 4, thence through infrared radiation-transmitting window 5 in housing 6, and impinges upon yarn 7. The feedstock yarn isunwound from supply reel 8 which is turned by an unreeling motor controlled by dancer 9. .The dancer consists of apulley which floats on the yarn and operates a microswitch. As downstream tension develops on the yarn, the dancer moves upward and starts the unreeling motor to supply more yarn and thereby maintain a predetermined tension, e.g., l'3,000 grams. On leaving housing 6, the carbonized or graphitized yarn passes over pulley l0 and is wound on reel 11. Housing 6 is provided with an entry port 12 for gases which may be non-oxidizing, e.g., nitrogen,

hydrogen, argon, helium, etc. to protect the hot fibers from oxidation, or may contain reactive gases or mixtures thereof, e.g., air, chlorine, hydrogen chloride, etc. to facilitate the carbonization or graphitization of the organic fibers. Housing 6 is also provided with a second infrared radiation-transmitting window 13 so that laser radiation which is not intercepted and absorbed by the yarn may be transmitted outside of the housing and subsequently harmlessly absorbed. Windows 5 and 13 are sealed against'housing 6 by O-rings 14 and suitable clamping means.

In FIG. 2 there is shown a modification in which a hollow cylindrical reflector directs the fraction of laser radiation not absorbed from the incident beam back onto the fibers. A condensing lens may or may not be used to concentrate the laser beam; in this embodiment it has been omitted. Thus, the collimated laser beam 21 emanates from laser tube 22 through radiation-transmitting lens 23. It passes through infrared radiationtransmitting window 24 in housing 25 and impinges upon yarn 26. Means are provided for moving yarn 26 at a controlled rate under tension through laser beam 21, as for example by the method illustrated in FIG. 1 or by simply employing a motor-driven take-up reel at the upper end of the yarn and by hanging a weight on the lower 'end. Cylindrical reflector 27 is a polished infrared radiation-reflecting hollow cylinder having a hole cut in its front wall for the entering beam. It is supported by rod 28 which passes through plate 29 and is manipulated for focusing and alignment by handle 30. For maximum utilization of the reflected energy the cylinder is positioned so that the yarn is in the focal plane, which is parallel to the rear wall and spaced therefrom by a distance equal to one-half of the radius of the' cylinder. Reflectors of other configurations, such as parabolic, can also be used for focusing the radiation the oxygen content was about 22 percent by difference.

onto the yarn. The yarn passes over yarn guides 31 which are rods, e.g., glass, graphite, Teflon, etc., equipped with smooth notches. Bottom plate 32 is provided with gas entry port 33. O-ring 34 are employed for seals as in FIG. 1.

DESCRlPTlON OF THE PREFERRED EMBODIMENTS The invention is further illustrated by, but not limited to, the following examples.

EXAMPLE 1 This example illustrates carbonization at a temperature below 1,200C.

The apparatus was essentially as represented in H6. 2 in which a reflector 27 having an internal diameter of 23 mm. was employed. The upper end of the yarn went to a motor-driven take-up reel. Tension was applied to the yarn by attaching a l0-gram weight to the lower end of yarn 26.

The feedstock consisted of preoxidized acrylonitrile homopolymer yarn having about 250 fibers in the bundle. For the preoxidation treatment the yarn was wrapped on a thin-walled glass cylinder taking care not to overlap the yarn. The yarn was then heated in an aircirculating oven in which the temperature was raised from 25to 280C. during 2 hours and thereafter held at 280C. for 3 hours. The now-blackened yarn was cooled, and washed in distilled water at the boil for 1 The laser beam was produced by a C0 laser made by Korad Corp., Model K-G3, wavelength 10.6;t, operating at up to about watts output.

The yarn was heated by two passes through the laser beam at an average temperature of about 1,060C. Residence time in the heated zone was about 1.6 minutes for each pass. Argon was blown through the housing.

The physical properties of the partially graphitized carbon fibers were determined on single fibers. For these as well as the products obtained in the remaining examples, the tests and method used were as follows: tensile strength and modulus were determined by the method of S. Schulman reported in the Journal of Polymer Science, Part C, Polymer Symposia, High Temperature Resistant Fibers,No. 19, pp. 211-225 (1967): Methods of Single Fiber Evaluation. The data reported here are the average of replicate determinations, usually six or more.

Physical Properties of Product I Tensile strength Elastic modulus EXAMPLE 2 This example illustrates the use of a condensing lens.

The apparatus consisted of a modification of that represented by FIG. 1: the yarn supply reel 8 and dancer 9 were removed and tension was applied to the yarn simply by attaching a 5-gram weight to the lower end of the yarn 7. A condensing lens 4 having a focal length of 6 inches was used.

The feedstock consisted of preoxidized acrylonitrile homopolymer yarn prepared as described in Example The yarn was passed repeatedly through the laser beam, increasing the power of the laser generator after each second pass so that the yarn attained the following observed temperatures (C): 820, 890, 1,010, 1,340, 1,890 and 1,930. The power output for the highest temperature was watts. During this operation, argon was blown through the housing. The yarn moved at a rate of 0.37 inches per minute. The residence time for the yarn in the heated zone was about 13.8 seconds for each pass when the lens was at a distance of 6.87 inches from the yarn. The yarn was rotated after the first pass at each temperature.

Physical Properties of Product ll Tensile strength Elastic modulus Elongation 134 X 10 p.s.i. 23.8 X 10 p.s.i. 0.57

EXAMPLE 3 The apparatus and method were essentially the same as in Example 2. The tension was applied by a IO-gram weight.

The feedstock consisted of preoxidized and precarbonized' acrylonitrile homopolymer yarn. The preoxidation conditions were the same as in Example 1. For the carbonization treatment, instead of using laser irradiation, the yarn was heated by a conventional, relatively low-temperature, method in a furnace at 950C. for about 6 minutes in a nitrogen atmosphere. The fibershad the following physical properties:

Tensile strength 89 X 10 p.s.i.

Elastic modulus 12.5 X 10 p.s.i.

Elongation 0.77

Laser irradiation was then applied to heating the yarn by successive passes through the laser beam, with two passes at-each of the following temperatures (C.): 1,160, 1,340, 1,950 and 1,990. At the highest temperature the 6-inch focal length condensing lens was at a distance of 6.87 inches. During the heating operation, argon was blown through the housing.

Physical Properties of Product III Tensile strength 118 X 10 p.s.i. Elastic modulus 26 X 10' p.s.i. Elongation 0.43

EXAMPLE 4 This example illustrates the use of a reflector, with stepwise heating in sevenv steps to a maximum of 1,420C.

The apparatus was essentially as represented in FIG. 2 in which a reflector 27 was employed. The upper end of the yarn went to a motor-driven take-up reel. Tension was applied to the yarn by attaching a lO-gram weight to the lower end of the yarn 26.The condensing lens was not used.

The feedstock consisted of preoxidized acrylonitrile homopolymer yarn prepared as in Example 1.

The yarn was heated by successive passes through the laser beam with two passes at each of the following temperatures (C.): ca. 400, ca. 500,890, 1,000, 1,210, 1,320 and 1,420. Argon was blown through the housmg.

Physical Properties of Product IV 164 X 10 p.s.i.

Tensile strength 26.2 X 10 p.s.i.

Elastic modulus EXAMPLE 5 Physical Properties of Product V-A 117 x 16 p.s.i.

Tensile strength 23.9 X 10' p.s.i.

Elastic modulus In Example 5-B, the yarn was heated by one pass at an average temperature of about 2,170C.

Physical Properties of Product V-B Tensile strength Elastic modulus EXAMPLE 6 This example illustrates heating to a maximum of about 2,270C.

The apparatus was the same as in Example 4. The feedstock was preoxidized acrylonitrile homopolymer prepared as in Example 1.

The yarn was heated by two passes through the laser beam at each temperature: first at about 1,030C., then atabout 2,270C. Residence time in the heated zone was about 1.6 minutes for each pass. Argon was blown through the housing.

The product was found by chemical analysis to be 97.76 percent carbon.

In another run, under similar conditions, a product was obtained with the following properties:

Physical Properties of Product VI-B Tensile strength Elastic modulus 112 X10 p.s.i. 30 X l0 p.s.i.

EXAMPLE 7 This example illustrates the use of a condensing lens and reflector in combination.

The apparatus consisted of the lasersource and con-v densing lens as represented in FIG. 1, and the reflector and housing as represented in; FIG. 2. The upper end of the yarn went to a motor-driven take-up reel. Tension was applied to the yarn by av IO-gram weight. The yarn moved at a rate of-0.37 inches per minute. The residence time for the yarn in the heated zone was about 66 seconds for each pass when the lens was at a distance of 10.5 inches.

The feedstock consisted of preoxidized acrylonitrile homopolymer yarn prepared as in Example 1.

The yarn was heated by successive passes through thelaser beam with two passes at each of the following temperatures I (C.): ca. 500, 1,000, 1,330, 1,530, 1,690, 1,930 and 2,080. Argon was blown through the housing.

Physical Properties of Product Vll Tensile strength Elastic modulus 139 x 10 p.s.i. 29.0 x 10 p.s.i.

EXAMPLE 8 Tensile strength X 10 p.s.i.

Elastic modulus 1.8 X 10 p.s.i.

Elongation 1.3

The yarn was heated by eight passes through the laser beam, as the temperature was raised in stages from l,520 to 2,500C.

Physical Properties of Product Vlll Tensile strength 80 X 10 p.s.i. Elastic modulus 8.2 X 10 p.s.i. Elongation 1.0

EXAMPLE 9 This example illustrates graphitizing the surface of a bonded fibrous body.

A layer of an epoxy resin-bonded carbon fiber composite is built up on ,a steel shaft by winding a carbon yarn liberally coated with curable epoxy resin. The composite is cured to a hard tough condition. The outer surface of the composite, which is substantially cylindrical in shape, is then exposed to laser radiation in an inert atmosphere. Upon increasing the level of irradiation gradually, to produce a maximum temperature of about 2,700C., the surface layers of carbon yarn are graphitized. Upon fitting the structure to a bearing, the shaft is found to turn smoothly because of the graphitized surface which results in a decreased coefficient of friction.

4 It is to be understood that although the invention has 1 been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are i fi 5 titi wherein R and R are selected from the group consisting of hydrogen, lower alkyl of up to three carbon atoms, phenyl, lower alkoxy containing up to three carbon atoms an itro and wherei the R rou s an be the same or 1 ererit and the R groups mu t be the same, and wherein X and Y are selected from the group consisting of hydrogen, lower alkyl containing up to three carbon atoms and phenyl, the phenylene radicals of said general formula being oriented other than ortho, having the steps of:

a. pretreating the fiber by heating at a temperature of from 180 to 550C. in an oxygen-containing atmosphere for a time sufficient to blacken the fiber;

b. heating the blackened fiber in a non-oxidizing atmosphere at a temperature between 700C. and about 1,200C. to carbonize the fiber; and thereafter heating the carbonized fiber in a non-oxidizing atmosphere at a temperature between about 1,200 and 3,600C. for a period of time greater than onetenth of a second to graphitize the carbonized fiber; wherein the improvement in the carbonizing and graphitizing heating steps comprises irradiating substantially all sides of the fiber at least once in a C0 laser beam the time of exposure being dependent upon the temperature.

2. In a method for preparing a graphitized fiber from a precursor fiber as disclosed in claim 1 having the steps of a. pretreating the fiber by heating at a temperature of from 180 to 550C. in an oxygen containing atmosphere for a time sufficient to blacken the fiber; and thereafter b. heating the blackened fiber in a non-oxidizing atmosphere at a temperature between 700C. and about 1,200C. to carbonize the fiber;

the improvement further comprising the further step of irradiating at least once substantially all sides of the carbonized fiber with a C0 laser beam in a non-oxidiz- 5 ing atmosphere at a temperature between about l,200

and 3,6'00C. for a period of time greater than onetenth of a second, said time being dependent upon the temperature.

3. A method of claim 1 in which the precursor fiber is within the full intended scope of this invention as acrylonitrile polymen defined by the appended claims.

What we claim is: 1. In a method for preparing a graphitized fiber from a precursor fiber selected from the group consisting of (I) acrylonitrile polymer and (2) an aromatic polyamide consisting of repeating units represented by the general formula:

4. A method of claim 1 in which the precursor fiber is an aromatic polyamide.

5. A method of claim 2 in which the precursor fiber is acrylonitrile polymer.

6. A method of claim 2 in which the precursor fiber is an aromatic polyamide. 

2. In a method for preparing a graphitized fiber from a precursor fiber as disclosed in claim 1 having the steps of a. pretreating the fiber by heating at a temperature of from 180* to 550*C. in an oxygen containing atmosphere for a time sufficient to blacken the fiber; and thereafter b. heating the blackened fiber in a non-oxidizing atmosphere at a temperature between 700*C. and about 1,200*C. to carbonize the fiber; the improvement further comprising the further step of irradiating at least once substantially all sides of the carbonized fiber with a CO2 laser beam in a non-oxidizing atmosphere at a temperature between about 1,200* and 3,600*C. for a period of time greater than one-tenth of a second, said time being dependent upon the temperature.
 3. A method of claim 1 in which the precursor fiber is acrylonitrile polymer.
 4. A method of claim 1 in which the precursor fiber is an aromatic polyamide.
 5. A method of claim 2 in which the precursor fiber is acrylonitrile polymer.
 6. A method of claim 2 in which the precursor fiber is an aromatic polyamide. 